This proposal concerns the determinants and biological role of the bacterial phospholipid degradation that accompanies the bactericidal action of polymorphonuclear leukocytes (PMN) and of potent membrane-active bactericidal proteins that recently have been purified in this laboratory. Our specific aims are to examine: I. The factors at determine differences among phospholipase A2 in their ability to act cooperatively with these purifief membrane-active proteins. II. The role of intramembrane CA++ in regulation of phospholipid hydrolysis. III. The effect of bacterial phospholipid degradation on digestion of bacteria killed by PMN. these aims have their origin in our earlier work and their pursuit will therefore rely heavily on well-tested methods used in this laboratory, including: assays for measuring the interaction of PMN with bacteria; use of bacterial mutants; assays for lipid biosynthetic and degradative activities; purification of proteins; chemical modification of proteins. Our long-term objectives, which relate to membrane biology and host-defense against infection, are to explore: 1) what regulates the action of defined intrinsic and exogenous phospholipases on the phospholipids of natural membranes. 2) the host's ability to digest microbial invaders and the reasons and pathophysiological consequences of incomplete digestion.